Absolutely correct, but typical automobile tires don't vary in diameter by enough to really swing that ratio too much one way or another, so I just say 2:1 to keep it nice and simple. My 225/75r15's actually calculate as having ever so slightly less than a 2:1 ratio.

But the tires typically found on Jeeps will vary from 30" to 37".

What's the difference (compared to each other) between adding 10lb to a 30" tire and adding 10lb to a 37" tire?

How significant is the added rotating mass when it is put on a Jeep that is already pushing the efficient/comfortable limits of it's motor and drivetrain? Would the drivability and effects of the additional rotating mass be exaggerated/more noticeable?

A little really sloppy math shows very little difference, SLADE. The thing about what you're asking about a 30 vs 37 (I used 30 vs 35) is that, assuming they're both on the same sized wheel (15" for example), all that really changes is that the sidewall grows by 2.5" and the tread grows by about 15" in circumference (that's only about a 16% growth). Everything about a 30" tire is still "inside" the 35" with just a little bit extra and only some of it held just a little bit farther from the axle.

Grrr, I typed this following paragraph about 7 times and couldn't really explain what I wanted to satisfactorily. The bottom line is that there's not much of a difference in the ratio. Whether or not the vehicle is regeared for the new tire size is extremely important, and that's why I just stop at figuring the ratio. When comparing a 30" to a 35", leverage/gearing is WAY more important a factor for comparison than equivalent mass. The equivalent mass ratio stays about the same, but because the bigger tire will be more massive, the effect will be slightly greater based on however much more massive it is.

From this month's Off-Road Adventures magazine: "Steel (wheel) is strong, cheap, easy to keep looking nice and most easily repaired. It's heavy and that contributes both to a loss of drivetrain durability due to increased weight/inertia and lower fuel economy, acceleration, and braking performance.."

Dang...they must be relying on common sense too. Shame on them. You guys go ahead and keep throwing around your numbers. We'll keep learning from experience on the road and trail.

That is soooo silly. Ankle weights compared to a back back. Is that the thing? Sort of like comparing carrying the groceries in your arms instead of in the cart isn't it?

On a Jeep wheel, every ft/lb of work a wheel does to rotate its mass upward on one side against the pull of gravity is offset by the downward motion of the other side pulling the wheel downward in rotation.

Now, Charley, if you could figure out a way for your left foot to pull up your right foot when your left foot is headed toward the ground, your analogy would be just perfect. But, alas, your legs are not connected to each other with some Rube Goldberg contraption. At least not in this world.

Charley, please don't assume that I have no real world experience with this. It's clear to me that many people refuse to even approach this from a scientific/mathematical level, and that's fine because this isn't easy math to grasp (even I have to take it slow and check and double/triple check myself every time to make sure I know what I'm doing), but I'm also a cyclist and have spent some years racing in addition to being a car guy. Rotational inertia is also a major player among cyclists (probably more so), and if you think that automotive geeks know a thing or two about rotating mass, you haven't met a bicycle "weight-weenie." You haven't seen anything until you've seen cycling weight-weenies fuss over a couple grams of mass here or there.

On that note, here is another source for exploring rotational inertia (it's actually linked on Mason's page as well) http://www.analyticcycling.com/WheelsInertia_Page.html. Yell "nah-uh" at me all you like, but this isn't the first thing in the history of science that people have stubbornly refused to believe because it's unintuitive, but that doesn't make it untrue. If you want to PROVE your own theory of the effect of rotational inertia, show your own calculations or results from a controlled experiment such as a pendulum test or some other reproducible/verifiable scientific method. Mason and I could be wrong, but I'm not about to give fuzzy analogies the time of day when pitted against real math.

On a scientific/mathematical level it's difficult to figure out, hard to grasp and according to the scientific/mathematical numbers there shouldn't be much of a difference between running a heavy tire/ wheel combo and a light tire/wheel combo.

Help me understand a few things...

Why is it so easy to feel the difference when your driving the vehicle and for some, see the difference in the fuel economy.

Why does the scientific/mathematical level and the seat of the pants level not correspond?

For the scientific/mathematical Ratio (2 to 1) you gave to be accurate, wouldn't you also need to take the gear ratio, motor horse power, and motor torque into consideration?

If the motor horse power/torque and the gear ratio isn't some how accounted for as a variable, how can you accurately come up with a ratio that represents the effects of the tires on a given vehicle?

OK Mschi. I'm sorry. I didn't mean to offend you. You're a good guy and you always have good intentions. We'll just have to disagree on the rotational mass topic, but I still think you're an awesome person, amd you've had a lot of great ideas and info in other threads on other topics.

So even though we disagree on this topic, I respect you and appreciate you, and you are good for the forum.

__________________
Warning: I often edit my posts a few times to get them complete, or to correct errors.

On a scientific/mathematical level it's difficult to figure out, hard to grasp and according to the scientific/mathematical numbers there shouldn't be much of a difference between running a heavy tire/ wheel combo and a light tire/wheel combo.

Help me understand a few things...

Why is it so easy to feel the difference when your driving the vehicle and for some, see the difference in the fuel economy.

Why does the scientific/mathematical level and the seat of the pants level not correspond?

For the scientific/mathematical Ratio (2 to 1) you gave to be accurate, wouldn't you also need to take the gear ratio, motor horse power, and motor torque into consideration?

If the motor horse power/torque and the gear ratio isn't some how accounted for as a variable, how can you accurately come up with a ratio that represents the effects of the tires on a given vehicle?

Excellent questions. It is very noticeable in real life, IME.

No offense to Mschi, but how do we know the guy who made the Website he references got it right? Perhaps he missed something. Lab experiments and physics story problems don't always reflect real life because sometimes some elememt of real life was not figured in.

All I know for sure is rotating mass matters a lot based on my experiences.

__________________
Warning: I often edit my posts a few times to get them complete, or to correct errors.

Wilson, you're consistently on the wrong side of almost every topic I've seen you participate in.

You're in no position to call anyone silly. You think common sense is silly. That's why someone called you a moron earlier in this thread, but I'd never do that because I'm to polite.

You're probably to old to know this, but there are now strap on ankle weights that wrap around ankles for wearing during excercise. They can be worn during running, aerobics, or other excercise.

However, they fell out of favor when people started suffering joint injuries from the extreme strain of wearing a weight on ankles. The reciprocating mass caused to much strain and injuries. So their popularity declined.

If a person wore a few of them on each ankle/leg, the tests I suggested could be done.

__________________
Warning: I often edit my posts a few times to get them complete, or to correct errors.

On a scientific/mathematical level it's difficult to figure out, hard to grasp and according to the scientific/mathematical numbers there shouldn't be much of a difference between running a heavy tire/ wheel combo and a light tire/wheel combo.

Help me understand a few things...

Why is it so easy to feel the difference when your driving the vehicle and for some, see the difference in the fuel economy.

Why does the scientific/mathematical level and the seat of the pants level not correspond?

For the scientific/mathematical Ratio (2 to 1) you gave to be accurate, wouldn't you also need to take the gear ratio, motor horse power, and motor torque into consideration?

If the motor horse power/torque and the gear ratio isn't some how accounted for as a variable, how can you accurately come up with a ratio that represents the effects of the tires on a given vehicle?

Usually this is a matter of placebo effect. Someone expects to feel an big impact, so they end-up thinking they feel one. This is why people's "butt dynos" are useless and never taken seriously when anyone talks about the changes a given mod make them feel. CAI mods are a great example of a place where often people will say they feel a big power increase but a true dyno run usually just says there's been little to no change. People's perceptions are unreliable. There can be other factors, but this is a major player and why I never take anecdotal "butt dyno" testimonies seriously...ever. Regarding tires, this is almost always confounded by the fact that when someone changes their tire weight, they almost always change tire model, diameter, and/or width at the same time. It's hard to tell what's causing what when you change multiple things at once. Tread patterns, compounds, and width can have a profound effect on resistance while different diameter, as all well-informed Jeepers know, don't have to change a whole lot to have big enough effects worthy of regearing.

No, the ratio is independent of gear ratios. It's an inertial equivalence. An object (wheel) has inertia whether or not it's attached to an engine and a bunch of gears, and those things don't change its mass or its inertia. They do change how the mass is moved of course. Lower gears make it easier for a given engine to accelerate a given amount of mass and all that etc etc.

Neither I nor Mason have shown the effects on varying masses on the vehicle. We've shown equivalent mass of rotating objects compared to static objects. The effects of either the rotating or static object on something like a vehicle is another can of math worms, but the ratio for the tire/wheel/driveshaft/whatever remains the same and becomes PART of those calculations.

No offense to Mschi, but how do we know the guy who made the Website he references got it right? Perhaps he missed something. Lab experiments and physics story problems don't always reflect real life because sometimes some elememt of real life was not figured in.

Those are always things to be asking in math/science. All I can say is that I've chugged the numbers and algebra on my own--albeit a bit sloppier than he--and ended-up with the same equations/results. I welcome disagreement, but it's gotta be backed-up somehow by math or science. I've chugged numbers to the best of my ability can came-up with something demonstrable, repeatable, and hopefully verifiable; that's what I'd like to see from anyone claiming otherwise. It's not a competition where I want to be right; that's not what science is about. I'd be super happy if someone came in here with some math or some data from a well-conducted controlled experiment that say something else.

Obviously we've both simplified things slightly. For example, we both approached tires as being composed of two discs (the sidewalls) and a band (the tread) and only guesstimated how the mass is distributed throughout the shape of the tire (about 2-3x more mass in the tread than the sidewalls; Mason's script calculator allows you to change this ratio, though).

We also both ignored the fact that a tire's diameter grows as it spins faster. Why, well, a tire's diameter also shrinks because weight pressing down on it and compressing it against the road. For simplicities sake, we both just left these attributes out of the equations figuring they likely cancel each other out to some degree.

Here's something I couldn't even fathom how to calculate, and all Mason says on the matter is that it's beyond the scope of his page/scripts: handling. Intertial mass (a spinning wheel as opposed to a static wheel) has some very noticeable effects on handling. An easy way to see what I'm talking about here is to take your bicycle wheel off your bike. Now hold it by the axle/skewer while it's NOT spinning and turn/tilt it. Easy, right? Spin it then tilt/turn it. Feel that? Yep, that's due to rotational inertia, and I don't have much of a clue how to start "mathing" that.